This invention relates to communications networks and in particular to a method and arrangement for facilitating communication in a TDMA network system, such as hybrid fibre/coax (HFC) networks employed in the distribution of cable television services.
A recent development in the telecommunications field has been the installation of hybrid fibre/coax networks e.g. for the delivery of cable television services to subscribers. These networks typically comprise a coaxial transmission system disposed between a group of subscribers and the system xe2x80x98head endxe2x80x99 where the coaxial system interfaces with a fibre optic transmission system. In order to generate increased revenue, the operators of these systems have allocated what would otherwise be spare bandwidth to a variety of services such as telephony, video, data and Internet services.
The use of HFC networks for services which can originate from a number of different service providers has necessitated the introduction of operating standards, for example the IEEE802.14 standard to ensure both uniformity of operation and compatibility with existing or legacy transmission systems.
The increasing use of compression in such networks will see voice coding schemes that can operate at bit rates as low as 4 kb/s emerging. It is important that the delivery system should be able to support these services efficiently together with additional communicative services such as multi-media and video-conferencing (e.g. the H323 video conferencing standard). An arrangement for providing communications in a distribution network is described in specification No WO-96-15599-A1. A discussion of techniques for supporting STM traffic with ATM is given by I Gard et al. in ISS ""95 World Telecommunications Congress Vol. 1, 23 April 1995, pages 62-66. Echo control in an ATM network is described by Z Tsai et al. in IEEE/ACM Transactions on networking, Vol. 2, No. 1, Feb. 1, 1994, pages 30-39. Techniques for providing communications in a distribution network are described by M Takefman in specification No. WO-96-15599.
A key driver in the development of the IEEE802.14 network standard has been the requirement to enable legacy 64 kb/s PCM services to be delivered over the HFC network. However there is an increasing trend towards the use of voice coding schemes that are both highly compressed and may additionally utilise speech activity detection to operate at bit rates as low as 4 kb/s. In addition to voice, H.323 interactive and MPEG2 distribution is a very powerful service combination enabled by current multimedia PCs. H.323 needs a multi-channel dynamic link to support, G.723 for voice, H.265 for video, H.245 for call handling and T.120 for data conferencing. There is thus a need to future proof the HFC delivery system so that it has the necessary capability to deliver these services in an efficient manner.
Voice quality is a key issue in the delivery of communicative services. The subjective quality is summarised for coding standards in Mean Opinion Scores. A key factor that influences these scores is distortion. Another factor influencing quality is echo signal delay. The ITU has also standardised on a combined delay and echo signal loss plan which is effective below a threshold of 25 ms. Above this threshold echo cancellation must be applied.
However echo cancellation introduces its own distortion. Once again the ITU limits the number of Quantisation Distortion Units (QDUs) on national and international links. It should be noted that even on an ideal 4 wire digital network, there would still be a need for echo cancellation, since all handsets suffer acoustic feedback, especially when brought close to the face. (In free air this is at best 50 dB signal loss, which is not better than the attenuation required at 25 ms mean one way propagation delay.)
It is therefore not feasible from a technical and cost standpoint to liberally spread echo cancellers around the network. Moreover, when interworking with legacy it is sometimes impossible to employ echo cancellation effectively, which should be as close to the far end or echo source as possible.
Clearly it is desirable to minimise unnecessary delay wherever possible. This can ensure as low an impact on existing network delay budgets and echo signal loss plans as possible and also avoid introducing complications and cost in planning newer networks.
These considerations have been a key driver in the definition of AAL-2. ATM packetisation represents a significant proportion of a network delay budget. For example, for legacy 64 kb/s PCM the ATM packetisation delay is approximately 6 ms which is equal to the total network propagation delay of most European national networks. The packetisation delay increases significantly as compression is introduced (e.g. 24 ms for 16 kb/s PCM). With a traditional single channel ATM adaptation layer paradigm, when the packetisation delay exceeds the budget allowance then the traditional option is to sacrifice bandwidth for delay by partially filling the payload. This, particularly at low bit rates can be extremely wasteful of bandwidth as a great deal of dummy information is transported and then discarded on receipt.
In systems employing asynchronous transfer mode (ATM) transmission, the ATM packetisation delay for communicative services represents a significant and sometimes unacceptable proportion of any delay budget. For example, for legacy 64 kb/s PCM services, the ATM packetisation delay (using AAL-1) is approximately 6 ms which is equal to the total network propagation delay of most national networks. The packetisation delay increases dramatically as the amount of compression increases (e.g. 24 ms for 16 kb/s ADPCM). With a traditional single channel ATM adaptation layer paradigm, when the packetisation delay exceeds the budgeted allowance, the traditional option is to sacrifice bandwidth for delay by partially filling the payloadxe2x80x94this, particularly at low bit rates may be extremely wasteful of bandwidth and hence the emergence of the AAL-2 paradigm which enables multiple low bit rate channels to be multiplexed into a single ATM payload.
An object of the invention is to minimise or to overcome this disadvantage.
It is a further object of the invention to provide an improved arrangement and method for the distribution of services over a hybrid fibre/coax network or other TDMA based shared access medium.
According to one aspect of the invention there is provided a method of transporting a communicative service over a TDMA or TDM network by packetising said services into minicells, and transporting the minicells in minislots defined by partition of TDM time slots.
According to another aspect of the invention there is provided method of transporting high and low bite rate communicative services in time slots over a time division multiple access (TDMA) network, the method comprising packetising said services into minicells for transmission within time slots allocated thereto, wherein said time slots are partitioned into a plurality of minislots each capable of accommodating one or more said minicells, and wherein said minislots are allocated in contiguous blocks for said high bit rate services and are allocated individually for said low bit rate services.
The minislots allocated to a low bit rate service may be allocated periodically from successive frames or on an as-needed basis.
According to another aspect of the invention, there is provided a distribution network for providing upstream and downstream high bit rate and low bit rate communicative services to a plurality of user terminals coupled thereto, the network incorporating a head end providing an interface to an ATM network in which user traffic is carried in minicells, wherein upstream communication in said distribution network is effected via a time division multiple access (TDMA) protocol defining a plurality of time slots each of which slots is partitioned into a plurality of minislots, and wherein the network is arranged to allocate said minislots in contiguous blocks for said high bit rate services and individually for said low bit rate services.
The access medium consists of a regular stream of TDMA structures termed mini-slots which are created by subdivision of TDMA time slots and which contain typically 8 bytes of payload data together with associated overhead information. The upstream slots can be concatenated together in order to deliver ATM cells (whole) or variable length fragmented messages. Concatenation of mini-slots is ideal to support the delivery of messaging or higher rate user data. For low bit rate communicative services, this concatenation in the transport layer replicates the packetisation delay incurred by a traditional ATM adaptation layer and is unsuited to the delivery of low bit rate communicative services. However, we have found that by allocating (for low bit rate communicative services only) mini-slots periodically rather than in the conventional blocks, the packetisation delay inherent within the transport layer can be significantly reduced. In particular, the preferred 8 byte mini-slot provides an ideal underlying transport structure even at the very lowest of bit rates and represents a reduction in the underlying packetisation delay of as much as 6 to 1 when compared to the traditional ATM payload. Further, the allocation of bandwidth in this manner can be undertaken at no adverse effect to alternative services.
Effectively, the rate at which minislots are allocated to a particular service is matched to the bit rate of that service so as to minimise packetisation delays while avoiding the need for redundant padding.
The minicells carried within the minislots may be of fixed or variable length.
Allocation of minislots may be performed at the system head end in response to service requests transmitted over a control channel from user terminals.
The use of short minislots together with the combination of minicells enables the greatest flexibility to be achieved in the compromise between low delay and high bandwidth utilisation.
The distribution network may carry multimedia traffic including voice, video and data facilities.